The german airlines introduced the 2 person rule after Germanwings, ultimately due to public pressure. A year later they quietly went back to the old procedure. Back then i was working in one of them and talked to the flight safety manager, he told me that the safety case was always against the 2 person rule, but public pressure was so high that they had to implement against their own safety case, which the authority was not happy about.
I believe some Big Airline never introduced it, and my current outfit finally dropped it in the middle of last year. So yes, there is quite a few airlines that do not require the 2 person rule.
As we have recently seen in the Atlas case, even a second pilot in the flightdeck does not prevent one pilot from bringing down the plane if he really wants to, certainly not a flight attendant on a jump seat.